Rolling stands comprising, as shown in FIG. 1, at least one pair of working or rolling rolls or cylinders 1, 1′ which are directly in contact with the strip during the rolling process, are used for the cold rolling of strips. One of the two working rolls vertically overlaps the other.
Such a configuration is limited in the forces applicable for the elastic deformation of the rolls themselves. In order to obviate this drawback, rolling stands comprising multiple rolls, including at least two rolling rolls 1, 1′ and two resting rolls 10, 10′ which oppose the elastic deformation of the rolling rolls 1, 1′, are used, which rolling rolls are intended to be in direct contact with the material to be rolled, as shown in the scheme in FIG. 1b. 
Other configurations of rolling stands are known from the prior art, where two rolls are working rolls, two rolls are intermediate rolls and two rolls are resting rolls. Configurations having multiple cylinders or rolls are also known, some being shown, as an example, in FIG. 1c, 1d, 1e. 
Every rolling stand is provided with various hydraulic actuators, including:                two hydraulic cylinders placed, for example, on the top of the stand or under the stand, and acting on the resting chocks for adjusting the distance between the rolling rolls, thereby controlling the thickness of the strip being rolled;        four or more hydraulic bending cylinders for each chock of the working rolls, defining a so-called bending control system which, acting on the chocks of the working rolls, change their elastic deformation allowing for the control of the planarity of the strip being rolled.        
The rolling force is applied on the necks of the resting rolls for controlling the thickness of the strip being rolled, while further forces are applied, by means of the bending control system, on the chocks of the working rolls in order to control the planarity of the strip being rolled.
The bending system is controlled through servo valves controlling the pressure within the chambers of the hydraulic bending cylinders in order to obtain the desired extent of elastic deformation for the working rolls.
The servo valves controlling the bending of the rolls have response times of the order of 50-200 ms with cut-off frequencies smaller than 50 Hz.
The rolling speed defines each single rolling mill capacity, since all the rolling mills basically try to roll for the maximum time possible at speeds which are next to the maximum speeds achievable by the drive train and allowed by the power installed in the plant.
During the rolling process, some forcing may be generated which, under certain conditions, may trigger resonances mainly in the vertical arrangement direction of the working rolls.
Such forcings may be generated by:                the strip itself, due to its intrinsic thickness or hardness variations;        friction variations within the rolling room, especially when reaching the limit speed resulting in the—even temporary—breakage of the lubricating film;        flaws induced in the working rolls during grinding operations;        inadequate conditions of the stand mechanics, such as wears, clearances between various components and damaged rolling bearings;        concurrently rolling hard material along with a strong thickness reduction and high rolling speed.        
Rolling stands, just like any mechanical element, have some peculiar resonance frequencies. If said forcings have frequencies which are close to or matching such peculiar resonance frequencies, some phenomena of vibration may be induced.
Such phenomena occur with a movement of the rolls, transversally to the rolling direction, i.e. occur vertically and may reach widths which cannot be controlled and are not adequate to the rolling process.
Such phenomena are known as chattering and may generate surface defects, such as light/dark strip markings or thickness variations resulting in the wasting of the rolled strip, the flaws depending on how the stand vibrates.
In order to avoid flaws or breakages of the strip being rolled, which may result in damages to the rolling stand, upon detection of a chattering phenomenon, the person in charge of controlling the rolling process usually reduces the rolling speed or applies damping procedures for such a phenomenon.
Two main types of chattering are known in the art as third- or fifth-octave vibrations.
The third-octave resonance occurs at frequencies from 100 to 200 Hertz, while those of the fifth-octave occur at frequencies from 500 to 700 Hertz.
Such phenomena are characterized by different vibration modes: a third-octave resonance induces a first vibration mode in which a working roll and the related resting roll move accordingly, while the upper and lower rolls vibrate in counter phase; a fifth-octave resonance induces a second vibration mode, in which the working rolls vibrate while the resting rolls are motionless.
When these resonance phenomena occur during the rolling process, the rolling speed may be decreased from 20 to 50% of the rolling mill design speed.
Chattering is therefore a significant problem affecting the operativeness of rolling mills because, besides causing the wasting of the product, significantly reduces its production capacity.
Considering the importance of this issue, the chattering phenomenon in the rolling process has been the subject of deep study and experimentation activities.
By the application of vibration sensors or velocimeters suitably mounted to the rolling stands, the triggering of a resonance phenomenon may be determined and signalized in order to anticipate the rolling mill deceleration as much as possible.
Such systems are currently used in a fully automatic manner and allow for a constant and continuous verification of the rolling mill vibration level, also promoting preventive maintenance schedules thereof.
Such systems allow to minimize the qualitative drop, but do not solve the problem related to the reduction in the rolling plant production capacity.
The manufacture of active or passive vibration damping systems has been the subject of study, in order to allow for rolling processes at speeds closer and closer to the rolling mill design speeds.
AT507087A4 discloses an apparatus and method for the semi-active reduction of pressure oscillations in a hydraulic system of a cold or hot rolling mill. In such document, “Semi-active reduction of pressure oscillations” means a reduction of the pressure oscillation width in a hydraulic system by means of a passive pressure oscillation damper, where the natural frequency of the passive damper may be changed by means of an actuator. The technical teaching of this document is to avoid using active vibration damping systems since the energy additionally introduced into the hydraulic system, through the actuator, importantly worsens the whole system stability, and may result in a spoilage in the response of the system. Particularly, the AT507087A4 solution provides for the pressure vibration reduction in a hydraulic line using a Helmholtz resonator. The system allows to dissipate the vibrational energy of the fluid in a chamber connected to the hydraulic line, different from the chamber of the hydraulic cylinder, thus reducing the pressure oscillations within the hydraulic line. The damping system is passive and is controlled to calibrate the system according to the correct operating frequency, modifying the chamber volume. The actuator simply changes the resonator volume without injecting more fluid in the hydraulic system. This variation of the volume changes the natural frequency of the oscillation damper, thereby adapting the natural frequency of the oscillation damper to the pressure oscillation frequency.
A further example of passive damping system is suggested in WO00/23204, where a piezoelectric actuator acts on the hydraulic fluid of a roll regulation system. The piezoelectric actuator is embedded in one of the walls of a pressure vessel of the hydraulic system, such that the actuator can carry out only one displacement of the hydraulic fluid, thus causing a variation of the pressure within the vessel which leads to a regulation of the multiple cylinder rolling mill. Also in this case, the actuator does not inject further fluid into the hydraulic system.